The invention relates to a steam soak process for recovering oil from a reservoir in which a relatively viscous oil is contained in a relatively steam impermeable layer overlying a water-containing layer which is much more permeable to steam.
Steam soak processes are known to be useful for recovering a relatively viscous oil or tar. For example, about 36 years ago British Pat. No. 511,768 described a "stop-cocking" process of injecting ssteam at a pressure exceeding the critical pressure of the oil, in hopes of avoiding a fractional distillation which might form pore-plugging residues of asphalt or coke, and then backflowing fluid from the reservior. British Pat. No. 911,889 described employing at least one steam injecting and fluid backflowing cycle followed by a steam drive between wells. U.S. Pat. No. 3,259,186 described a steam soak process in which the steam is injected at a pressure kept below the overburden pressure. Various U.S. Pat. Nos. e.g., 3,333,637; 3,349,849; 3,354,958; 3,358,762; 3,409,083 and 3,455,392 have suggested additional variations in steam soak oil recovery processes.
Various problems are particularly troublesome in producing oil from a viscous oil reservoir in which an oil layer overlies a water layer that is more permeable to steam. Such reservoirs and problems are discussed in the following U.S. patents, although the patents relate to steam drive oil recovery processes. U.S. Pat. No. 3,439,742 suggests circulating hot water and steam through a water layer so that heated oil will be entrained and produced without a need for fracturing the oil layer. U.S. Pat. No. 3,682,244 suggests that each production well be plugged back when a steam breakthrough into its lower portion becomes imminent. U.S. Pat. No. 3,847,219 suggests that the injection and production rates first be adjusted to develop and maintain a relatively high steam pressure and later be gradually reduced, during a blowdown cycle of enhanced oil production.